An electroluminescent device includes an inorganic electroluminescent device and an organic electroluminescent device. The inorganic electroluminescent device includes a thin film type and a dispersion type both of which generally require a high AC voltage having several tens of V or more for emitting light. On the other hand, the organic electroluminescent device has an effect that it can emit light with high luminance at a DC voltage of 10 V or less.
The organic electroluminescent device is generally formed of a luminescent body (organic electroluminescent body) comprising a transparent electrode, an organic luminescent layer, and a metal electrode which are laminated in that order on a transparent substrate.
The organic luminescent layer is formed of a laminated body of various organic thin films, for example, there is known a body having various combinations of layers such as a laminated layer formed of a hole injection layer made of triphenylamine derivative and a luminescent layer made of fluorescent organic solid such as anthracene, a laminated layer formed of such a luminescent layer and an electron injection layer made of perylene derivative, and a laminated layer formed of a hole injection layer, the luminescent layer and the electron injection layer set forth above.
The organic electroluminescent device emits light based on the following principle. That is, when the voltage is applied to the transparent electrode and metal electrode, holes and electrons are injected into the organic luminescent layer, wherein the holes and electrons are recombined to generate energy, which energy excites the fluorescent material, and the organic electroluminescent device emits light when the excited fluorescent material returns to the ground state.
The mechanism of recombination of the holes and electrons in the course of emitting light is similar to that of ordinary light emitting diodes, so that the current and also the luminescent intensity exhibit non-linear characteristics with respect to the applied voltage. This results in rectification characteristics of the organic electroluminescent device.
In the organic electroluminescent device, at least one of the electrodes must be transparent for permitting light to travel outside, so that the transparent electrode formed of a transparent conductor made of indium tin oxide (ITO) is generally employed as an anode.
To enhance luminous efficiency by facilitating the injection of electrons, it is important to employ material having a lower work function as a cathode, and hence a metal electrode made of Mg--Ag, or Al--Li is normally employed.
In such an organic electroluminescent device having the arrangement set forth above, the organic luminescent layer is formed of a very thin film having a thickness of about 100 nm. Accordingly, the organic luminescent layer as well as the transparent electrode is substantially transparent. Consequently, when the organic electroluminescent device is turned off, environmental light transmits through the transparent substrate, the transparent electrode and the organic luminescent layer, then it is reflected by the metal electrode, and successively is ejected from the front surface of the transparent substrate. As a result, the display area of the organic electroluminescent device looks like a mirror.
In case that the organic electroluminescent device is employed as a light emitting element or a display element, it is very unnatural for the display area to look like a mirror when the device is turned off, and hence the use of this device is remarkably limited. This problem becomes particularly serious under conditions of a bright environment.